The campaign director behind the recent success of New Zealand Labour is facing calls to resign after he failed to tell the police, parents or the prime minister about the alleged sexual assault of four teenagers at a summer camp held by the party.

Jacinda Ardern was kept in the dark after the young Labour camp last month, said the party’s general secretary Andrew Kirton, for fear the victims would be further traumatised by more people knowing.

The teenagers’ parents were also not made aware and no formal support was offered by the Labour party to the victims until three weeks afterwards.

According to Newsroom, the young Labour members – two boys and two girls, all aged 16 – said a 20-year-old man sexually assaulted them at the weekend camp in early February. Three of the four victims said he put his hands down their pants.

Kirton said the alleged offender was ejected and banned from future party events. But news of the incident initially went no further than the inner circles of Labour, where it was decided to take a “survivor-led response”.

“The reason why I didn’t [tell the prime minister] is the advice I’ve got is it could add more distress when a wider group of people became aware of the incident – we wanted to make sure to protect the confidentiality of those involved,” Kirton told Radio New Zealand.

“We decided not to expand the concentric circles of people who know about this, it can be very traumatic and embarrassing for those that are the victims of this … the advice I had is these young people need to make the choices themselves about who they tell.

“It might be something they want to put behind them.”

Senior cabinet minister Megan Woods appears to have been the only government figure who knew of the allegations, having been contacted on Facebook by one of the victims. She also did not tell the prime minister.

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Ardern, who had spoken at the camp earlier in the weekend, was caught off guard by the allegations at her weekly media stand-up on Monday afternoon. She told Radio New Zealand on Tuesday that she had “no problem” with not being told but the three-week delay in offering support services to the four victims was “wrong”.

“I have to think of this from a personal perspective and if it were me and I were the parent, I would want to know,” said Ardern, referring to Kirton’s decision not to inform the parents of the four children involved.

“I am not going to say that what we did was appropriate in response … We should have brought in the professionals straight away, everyone acknowledges that now.”

Assistant commissioner of police investigations Richard Chambers said officers were aware of media reports of the alleged sexual assaults and encouraged anyone with information to come forward.

Dr Melanie Beres, a senior lecturer in sociology at the University of Otago and expert in sexual violence prevention, said Labour had followed best practice in supporting the victims to make their own decisions.

“It is really tough, with them being 16, for the parents in particular to feel they should have been told, but the ways in which it was handled are actually quite consistent with best practice in that sense,” said Beres.

A second allegation surfaced on Tuesday. Kirton said a woman had contacted him alleging she had been sexually assaulted at a different young Labour event within the last 10 years. The woman said she had been offered support and help going to police, Stuff reported.